Re: [PATCH 3/3] KVM: selftests: Check that KVM_FEATURE_PV_UNHALT is cleared with KVM_X86_DISABLE_EXITS_HLT

[Sean Christopherson <seanjc@google.com>]

Shortlog is a wee bit long, I went with:

  KVM: selftests: Check that PV_UNHALT is cleared when HLT exiting is disabled

On Wed, Feb 28, 2024, Vitaly Kuznetsov wrote:
> KVM_FEATURE_PV_UNHALT is expected to get cleared from KVM PV feature CPUID
> data when KVM_X86_DISABLE_EXITS_HLT is enabled. Add the corresponding test
> to kvm_pv_test.
> 
> Note, the newly added code doesn't actually test KVM_FEATURE_PV_UNHALT and
> KVM_X86_DISABLE_EXITS_HLT features.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---

...

> +	TEST_ASSERT(ent->eax & (1 << KVM_FEATURE_PV_UNHALT),
> +		    "Enabling X86_FEATURE_KVM_PV_UNHALT had no effect");
> +
> +	/* Make sure KVM clears vcpu->arch.kvm_cpuid */
> +	ent = vcpu_get_cpuid_entry(vcpu, KVM_CPUID_SIGNATURE);
> +	kvm_sig_old = ent->ebx;
> +	ent->ebx = 0xdeadbeef;
> +	vcpu_set_cpuid(vcpu);
> +
> +	vm_enable_cap(vm, KVM_CAP_X86_DISABLE_EXITS, KVM_X86_DISABLE_EXITS_HLT);
> +	ent = vcpu_get_cpuid_entry(vcpu, KVM_CPUID_SIGNATURE);
> +	ent->ebx = kvm_sig_old;
> +	vcpu_set_cpuid(vcpu);
> +	ent = vcpu_get_cpuid_entry(vcpu, KVM_CPUID_FEATURES);
> +
> +	TEST_ASSERT(!(ent->eax & (1 << KVM_FEATURE_PV_UNHALT)),

X86_FEATURE_KVM_PV_UNHALT already exists, all we're missing is a helper to get
a CPUID feature from host userspace given a vCPU.  I added this

	static inline bool vcpu_cpuid_has(struct kvm_vcpu *vcpu,
					  struct kvm_x86_cpu_feature feature)
	{
		struct kvm_cpuid_entry2 *entry;

		entry = __vcpu_get_cpuid_entry(vcpu, feature.function, feature.index);
		return *((&entry->eax) + feature.reg) & BIT(feature.bit);
	}

and used it in this test instead of open coding the reg+bit.